The present invention relates to an inductive displacement sensor.
An inductive displacement sensor of the before-mentioned kind has been disclosed, for example, in DE 100 16 540 C1 and FR 2 682 760 A1. The disclosed displacement sensor comprises an electrically insulated rectangular substrate with an inductor arranged thereon, which can be displaced along the substrate, which embraces the substrate in the form of a U and which serves as a displacement measuring head.
The measuring head consists of a ferromagnetic material. An excitation loop of rectangular shape and a measuring loop of V-shape are provided on the substrate. When an alternating voltage is applied to the excitation loop, it induces, according to Faraday's Law, an alternating voltage in the measuring loop across the magnetic circuit forming in the measuring head and across air. Due to the geometric shape of the measuring loop, which varies continuously along the displacement path of the measuring head, and due to the shape of the excitation loop, that does not change over the displacement path, the output signal of the measuring loop likewise varies continuously as the measuring head is displaced along the substrate.
DE 39 13 861 A1 describes a similar displacement sensor, but with a measuring head that comprises an electrically active (inductive) element which is part of a resonant circuit that can be excited from the outside. Preferably, the inductive element consists of a core coil with a magnetically soft core of substantially E-shaped configuration. The center limb of that core coil is arranged to extend above the measuring and reference loop.
In addition to the measuring loop, the displacement sensor described in DE 39 13 861 A1 comprises a reference loop of, preferably, rectangular shape which serves to observe a control voltage of a resonant circuit that can be acquired from the variations in flux density occurring, for example, as a result of voltage variations of the alternating voltage source or variations in ambient temperature. This permits the flux density of the magnetic field produced by the inductive element to be kept at a constant level. The inductive element induces a measuring voltage in the measuring loop and a reference voltage in the reference loop. For keeping the reference voltage at a constant level, the alternating voltage exciting the resonant circuit can be adjusted to a predetermined threshold voltage by means of a P or a PI controller, depending on the reference voltage.
A displacement sensor comprising a reference loop of the before-mentioned kind is disclosed also in unpublished German Patent Applications Nos. 101 20 822.7 and 100 16 540.0. In the case of those arrangements, both the measuring loop and the reference loop are used as evaluating loops, the measuring signal being derived as the quotient of measuring voltage and reference voltage. By forming the quotient it is possible, especially, to arithmetically eliminate any temperature drifts and positional variations of the measuring head.
In most of the cases, the before-mentioned measuring head is connected to an external evaluation circuit via a conductor, or without a conductor, for example by optical means or a radio link. The evaluation circuit serves for computing the respective displacement distance of the inductor on the substrate as a function of the measuring voltage determined and, if desired, in addition to the before-mentioned correction by means of the reference loop.
It should be noted in this connection that the reference loop and the measuring loop can be arranged on substrates of any desired shape, or even without a substrate, as long as no-contact sampling by the measuring head remains possible. Preferably, a printed circuit board is used as a substrate.
Now, it is a disadvantage of the displacement sensor described in DE 39 13 861 A1 and DE 100 16 540.0 that the measuring head is of the active type so that, consequently, it requires a permanent energy supply which can be realized only by lines that move together with the head.
The measuring arrangement described in FR 2 682 760 A1 is connected with the disadvantage that temperature influences and positional variations of the displacement sensor may lead to unstable measured values. In addition, a relatively high operating power is rendered necessary by the permanent excitation required if useful signals are to be obtained.
In addition, the displacement sensors known from the prior art are, on principle, connected with the problem known as the “air coupling” phenomenon. The air coupling phenomenon is due to electric coupling, according to Faraday's Law, between the excitation loop and the measuring loop(s) outside the active region of the magnetizable core of the measuring head.